1. Field of the Invention
The present invention is directed to a method for mapping color values of a color image onto the color values of a highlight-color image (HC image). In particular, the invention is directed to a method wherein the color values of the color image are represented as vectors of the RGB (Red, Green and Blue) color space, and the color values of the highlight-color image only comprise vectors of a highlight-color color space that covers the region in the RGB color space between all gray scale levels and a highlight-color color.
2. Description of the Related Art
U.S. Pat. No. 5,237,517 discloses a method for mapping color values of a color image onto the color values of a highlight-color image that covers only color values of a highlight-color color space. Such a highlight-color color space comprises all color values in the region between the gray scale levels and a predetermined highlight-color color. This method is also disclosed in “Principles for Mapping from Full Color to Highlight-color”, Steven J. Harrington, in IS&T's Eighth International Congress on Advances in Non-Impact Printing technologies, 1992, pages 458 ff.
In this method, the color values of the color image, which are usually represented as RGB color values, are subjected to a coordinate transformation into YES color values. This is a specific color system wherein the fully saturated color values are arranged in a plane. The YES coordinate system comprises three coordinates Y, E, S, whereby Y is the brightness of the color value and E and S are respective color values that define a color plane. When mapping the color values of the color image onto the highlight-color color space, the respective color values are first projected into the color plane. The color value of the highlight-color color is also projected into the color plane, whereby the projected color value of the color value to be mapped is projected anew onto this projected color value. This projection ensues by means of a calculation of the cosine between the two projected color values, whereby this value is raised to the power of three. This exponentiation of the cosine value is expedient since the individual, fully saturated colors (red, yellow, green, cyan, blue, magenta) describe an angle of 60° in the color plane with the respectively adjacent color, so that mapping without raising to the third power would map half the amount onto two neighboring primary colors since the cosine of 60° is equal to ½. The mapping of such a large amount of a primary color onto another primary color, however, is undesired since this has an unnatural effect. Due to the exponentiation with three, the amount of the mapping is reduced to ⅛ in the mapping for the angle of 60°. It is also advantageous that the operational sign is retained in the exponentiation with three.
The color values mapped in this way onto the highlight-color space serve the purpose of driving the HC color of a printer or color picture screen. Since a color picture screen is usually driven with RGB color values, the mapped color values must be subjected to another coordinate transformation into the RGB color space.